U.S. patent number 6,489,391 [Application Number 09/509,285] was granted by the patent office on 2002-12-03 for process for preparing aqueous, protective colloid stabilized vinyl ester homopolymer and copolymer dispersions.
This patent grant is currently assigned to Wacker-Chemie GmbH. Invention is credited to Georg Riepl, Bernd Schilling.
United States Patent |
6,489,391 |
Schilling , et al. |
December 3, 2002 |
Process for preparing aqueous, protective colloid stabilized vinyl
ester homopolymer and copolymer dispersions
Abstract
The invention provides a method of producing aqueous, protective
colloid stabilized vinyl ester homo- and co-polymer dispersions
with a defined solids content by emulsion polymerization in the
presence of a protective colloid. The method specifies that a
dispersion with a solids content lower than the defined solids
content is produced and then thickened to the defined solids
content after the polymerization has been completed. The dispersion
is thickened by adding a polymer powder which is of the same
polymer composition as the emulsion polymerizate and which can be
redispersed in water.
Inventors: |
Schilling; Bernd (Macungie,
PA), Riepl; Georg (Emmerting, DE) |
Assignee: |
Wacker-Chemie GmbH (Munich,
DE)
|
Family
ID: |
7843809 |
Appl.
No.: |
09/509,285 |
Filed: |
May 22, 2000 |
PCT
Filed: |
September 17, 1998 |
PCT No.: |
PCT/EP98/05919 |
PCT
Pub. No.: |
WO99/16826 |
PCT
Pub. Date: |
April 08, 1999 |
Foreign Application Priority Data
|
|
|
|
|
Sep 26, 1997 [DE] |
|
|
197 42 678 |
|
Current U.S.
Class: |
524/503; 156/332;
524/459 |
Current CPC
Class: |
C08J
3/005 (20130101); C08L 31/02 (20130101); C09J
131/02 (20130101); C08L 31/02 (20130101); C08J
2331/04 (20130101); C08L 2201/50 (20130101); C08L
2205/02 (20130101); C08L 2205/18 (20130101); C08L
2666/04 (20130101) |
Current International
Class: |
C08L
31/02 (20060101); C08L 31/00 (20060101); C09J
131/02 (20060101); C08J 3/00 (20060101); C09J
131/00 (20060101); C08L 029/04 () |
Field of
Search: |
;524/459,503 ;156/332
;427/207.1,280,342 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
37 24331 |
|
Feb 1989 |
|
DE |
|
0 567 812 |
|
Nov 1993 |
|
EP |
|
0 614 922 |
|
Sep 1994 |
|
EP |
|
0 708 122 |
|
Apr 1996 |
|
EP |
|
1158088 |
|
Jul 1969 |
|
GB |
|
96/11234 |
|
Apr 1986 |
|
WO |
|
Other References
Derwent Abstract corresponding to De 37 24331
[AN-1989-040026]..
|
Primary Examiner: Reddick; Judy M.
Attorney, Agent or Firm: Brooks & Kushman P.C.
Claims
What is claimed is:
1. A process for manufacturing a protective colloid-stabilized
vinyl ester homopolymer or copolymer product dispersion of a
specified polymer solids content in the range of 50 to 75 weight
percent solids based on the weight of the dispersion, and a
Brookfield viscosity measured at 20 rpm of from 20,000 mPas to
60,000 mPas, said process comprising the steps of: a) emulsion
polymerizing one or more vinyl ester monomers and, in the case of
vinyl ester copolymers, one or more vinyl ester monomers and one or
more non-vinyl ester comonomers, in the presence of a protective
colloid, to form a first protective colloid-stabilized vinyl ester
homopolymer or copolymer dispersion, and spray drying said first
protective colloid-stabilized vinyl ester homopolymer or copolymer
dispersion to form a water-redispersible polymer powder; b)
emulsion polymerizing the same vinyl ester monomers and non-vinyl
ester comonomers employed in step a) in the presence of a
protective colloid to form a second protective colloid-stabilized
vinyl ester homopolymer or copolymer dispersion having a Brookfield
viscosity measured at 20 rpm of from 10,000 mPas to 15,000 mPas and
a solids content lower than said specified solids content and in
the range of 30 to 65 weight percent based on the weight of said
second protective colloid-stabilized vinyl ester homopolymer or
copolymer dispersion; and c) adding to said second protective
colloid-stabilized vinyl ester homopolymer or copolymer dispersion,
sufficient of said water-redispersible polymer powder to increase
the solids content of said second protective colloid-stabilized
dispersion to the specified solids content of said vinyl ester
homopolymer or copolymer product dispersion, and to increase the
viscosity to the range of from 20,000 mPas to 60,000 mPas.
2. The process of claim 1 wherein said protective colloid comprises
a polyvinyl alcohol containing from 75 to 100 mol percent of vinyl
alcohol units and a Hoppler viscosity of from 10 to 25 mPas
measured in 4% by weight aqueous solution in accordance with DIN
53015.
3. The process of claim 1 wherein the solids content of the
protective colloid-stabilized vinyl ester homopolymer or copolymer
dispersion b) is from 40% by weight to 50% by weight based on the
weight of the dispersion.
4. The process of claim 1 wherein said vinyl ester homopolymer or
copolymer dispersions a) and b) are prepared by polymerizing
monomer(s) or monomer mixtures selected from the group consisting
of vinyl acetate; vinyl propionate; vinyl ester(s) and ethylene
with an ethylene content in the polymer of 1 to 60 weight percent
based on the weight of the polymer; and vinyl ester(s), ethylene,
and vinyl chloride, with an ethylene content of 1 to 40 weight
percent and a vinyl chloride content of 20 to 90 weight percent,
these weight percents based on the weight of said homopolymer or
copolymer.
5. The process of claim 1 wherein the homopolymer or copolymer
dispersions a) and b) are both vinyl acetate homopolymer
dispersions.
6. The process of claim 1, wherein the homopolymer or copolymer
dispersions a) and b) are both "vinyl acetate-vinyl propionate
copolymer dispersions".
7. A process for manufacturing a protective colloid-stabilized
vinyl ester homopolymer or copolymer product dispersion of a
specified polymer solids content in the range of 50 to 65 weight
percent solids based on the weight of the dispersion, and a
Brookfield viscosity measured at 20 rpm of from 20,000 mPas to
60,000 mPas, said process comprising the steps of: a) emulsion
polymerizing one or more vinyl ester monomers and, in the case of
vinyl ester copolymers, one or more vinyl ester monomers and one or
more non-vinyl ester comonomers, in the presence of a protective
colloid, to form a first protective colloid-stabilized vinyl ester
homopolymer or copolymer dispersion, and spray drying said first
protective colloid-stabilized vinyl ester homopolymer or copolymer
dispersion to form a water-redispersible polymer powder; emulsion
polymerizing the same vinyl ester monomers and non-vinyl ester
comonomers employed in step a) in the presence of a protective
colloid to form a second protective colloid-stabilized vinyl ester
homopolymer or copolymer dispersion having a Brookfield viscosity
measured at 20 rpm of from 10,000 mPas to 15,000 mPas and a solids
content lower than said specified solids content and in the range
of 40 to 50 weight percent based on the weight of said second
protective colloid-stabilized vinyl ester homopolymer or copolymer
dispersion; and c) adding to said second protective
colloid-stabilized vinyl ester homopolymer or copolymer dispersion,
sufficient of said water-redispersible polymer powder to increase
the solids content of said second protective colloid-stabilized
dispersion to the specified solids content of said vinyl ester
homopolymer or copolymer product dispersion, and to increase the
viscosity to the range of from 20,000 mPas to 60,000 mPas.
8. In the process of adhesively bonding of porous substrates
wherein an aqueous polymer dispersion is employed as an adhesive,
the improvement comprising bonding said porous substrates with a
protective colloid-stabilized vinyl ester homopolymer or copolymer
product dispersion prepared by the process of claim 1.
9. The process of claim 8 wherein at least one porous substrate is
wood, and wherein at least one water soluble salt of a monobasic
acid is added to said protective colloid-stabilized vinyl ester
homopolymer or copolymer dispersion as a crosslinker in an amount
effective to increase the water resistance of the adhesive as
compared to an adhesive of the same polymer content not containing
said crosslinker.
10. The process of claim 9 wherein said crosslinker is selected
from the group consisting of aluminum (III) salts, zirconium (IV)
salts, and mixtures thereof.
Description
BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to a process for preparing aqueous,
protective colloid stabilized vinyl ester homopolymer and copolymer
dispersions of defined solids content.
2) Background Art
Aqueous, polyvinyl alcohol stabilized dispersions of vinyl ester
homopolymers or vinyl ester copolymers are used in particular as
adhesives.
Dispersions having particular defined solids contents are required
depending on the application in which these dispersions are to be
used. Since the solids content of a polymer dispersion cannot be
controlled precisely during the polymerization, and can only be
increased even after the end of the polymerization by means of
technically complex measures, the removal of water in vacuo being
one example, which increases the production costs by prolonging the
preparation times, the procedure adopted in practice is to prepare
polyvinyl ester dispersions with a solids content higher than that
required and then dilute the dispersion to the required solids
content by adding water.
A problem with this "water dilution process" is that in the range
of solids contents which is relevant in practice, of between 40%
and 60%, the viscosity of polyvinyl alcohol stabilized polyvinyl
ester dispersions does not rise linearly with the solids content;
in this range, rather, the viscosity of the dispersions increases
exponentially. A consequence of this is that, in the polymerization
of dispersions having a solids content higher than that to be
established subsequently by water dilution, polymerization is
carried out with polymerization batches which are of relatively
high viscosity. The relatively high viscosity hinders the
dissipation of heat by way of the reactor walls, so necessitating
the installation of additional cooling equipment such as reflux
condensers, leading to the formation of wall deposits, and,
furthermore, markedly increasing the polymerization times. A
further disadvantage is that the removal of residual monomers from
dispersions of relatively high viscosity is made more
difficult.
DE-A 3724331 discloses a process in which the viscosity of aqueous
polyvinyl ester dispersions is increased by adding pulverulent
copolymers of vinyl alcohol-1-methylvinyl alcohol copolymers. With
these rheological additives it is, indeed, possible to increase
markedly the viscosity of the dispersions with a virtually
unchanged solids content; however, this rheological additive does
alter the performance properties of the dispersions. For example,
even low-level additions of these additives, which are readily
soluble in cold water, impair the water resistance.
In order to increase the solids content it is proposed in a number
of patent applications, for example, EP-A 614922 (US-A 5430092),
EP-A 567812 (US-A 5340858), EP-A 708122, and WO-A 96/11234, to
prepare the polymer dispersions in the presence of seed latices
which are included in whole or in part in the initial charge or are
metered in their entirety, giving polymer dispersions having a
biomodal particle size distribution. A disadvantage of this
procedure is the predominantly highly complex process regime.
Furthermore, although dispersions of high solids content are
obtainable by means of this procedure, this solids content also can
no longer be increased subsequently.
SUMMARY OF THE INVENTION
The object was therefore to provide a process for preparing
aqueous, protective colloid stabilized polyvinyl ester dispersions
by means of which polymer dispersions of defined solids content are
obtainable without the occurrence of the abovementioned
disadvantages of the water dilution process, without the need for
complex process steps, and without adversely affecting the
performance properties of the polymer dispersions.
The invention provides a process for preparing aqueous, protective
colloid stabilized vinyl ester homopolymer and copolymer
dispersions of defined solids content by emulsion polymerization in
the presence of protective colloid, characterized in that it
comprises preparing a dispersion with a solids content lower than
the defined solids content and, after the end of the
polymerization, thickening the dispersion to the defined solids
content by adding water-redispersible polymer powder of the same
polymer composition, and prepared by the same process with
subsequent drying, as the emulsion polymer.
The procedure of the invention is suitable for polymerizing one or
more monomers from the group of the vinyl esters of unbranched or
branched alkylcarboxylic acids having 1 to 15 carbon atoms, or
mixtures of said vinyl ester monomers with one or more comonomers
from the group of the methacrylic esters and acrylic esters of
unbranched or branched alcohols having 1 to 12 carbon atoms, the
fumaric and maleic monoesters or diesters of unbranched or branched
alcohols having 1 to 12 carbon atoms, the dienes such as butadiene
or isoprene, the olefins such as ethene or propene, the vinyl
halides such as vinyl chloride.
Preferred vinyl esters are vinyl acetate, vinyl propionate, vinyl
butyrate, vinyl 2-ethylhexanoate, vinyl laurate, 1-methylvinyl
acetate, vinyl pivalate and vinyl esters of alpha-branched
monocarboxylic acids having 5 to 11 carbon atoms, for example,
VeoVa9.RTM. or VeoVa10.RTM. (trade names of Shell). Vinyl acetate
is particularly preferred.
Preferred methacrylic esters or acrylic esters are methyl acrylate,
methyl methacrylate, ethyl acrylate, ethyl methacrylate, propyl
acrylate, propyl methacrylate, n-butyl acrylate, t-butyl acrylate,
n-butyl methacrylate, t-butyl methacrylate, and 2-ethylhexyl
acrylate.
Preferred ester groups of the fumaric and maleic acid are methyl,
ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, t-butyl, hexyl,
ethylhexyl, and dodecyl groups.
If desired it is also possible to copolymerize from 0.05 to 10.0%
by weight of auxiliary monomers, based on the overall weight of the
monomer mixture. Examples of auxiliary monomers are ethylenically
unsaturated carboxylic acids, preferably acrylic acid or
methacrylic acid; ethylenically unsaturated carboxamides,
preferably acrylamides; ethylenically unsaturated sulfonic acids
and/or their salts, preferably vinylsulfonic acid,
2-acrylamido-2-methylpropanesulfonic acid. Further examples are
precrosslinking comonomers such as polyethylenically unsaturated
comonomers, for example, divinyl adipate, diallyl maleate, allyl
methacrylate or triallyl cyanurate, or postcrosslinking comonomers,
for example, acrylamidoglycolic acid (AGA), methacrylamidoglycolic
acid methyl ester (MAGME), N-methylolacrylamide (NMA),
N-methylolmethacrylamide, N-methylolallylcarbamate, alkyl ethers,
such as the isobutoxy ether, or esters of N-methylolacrylamide, of
N-methylolmethacrylamide or of N-methylolallylcarbamate.
Examples of preferred vinyl ester polymers which comprise if
desired said auxiliary monomer units in the stated amounts are:
vinyl acetate and vinylpropionate polymers; vinyl ester-ethylene
copolymers such as vinyl acetate-ethylene copolymers having an
ethylene content of from 1 to 60% by weight; vinyl
ester-ethylene-vinyl chloride copolymers having an ethylene content
of from 1 to 40% by weight and a vinyl chloride content of from 20
to 90% by weight, vinyl esters present being preferably vinyl
acetate and/or vinylpropionate and/or one or more copolymerizable
vinyl esters such as vinyl laurate, vinyl pivalate, vinyl
2-ethylhexanoic acid ester, vinyl esters of an alpha-branched
carboxylic acid, especially Versatic acid vinyl esters; vinyl
acetate copolymers with from 1 to 50% by weight of one or more
copolymerizable vinyl esters such as vinyl laurate, vinyl pivalate,
vinyl 2-ethylhexanoic acid ester, vinyl esters of an alpha-branched
carboxylic acid, especially Versatic acid vinyl esters
(VeoVa9.RTM., VeoVa10.RTM.), which if desired also contain from 1
to 40% by weight of ethylene; vinyl ester-acrylic ester copolymers
with from 30 to 90% by weight of vinyl ester, especially vinyl
acetate, and from 1 to 60% by weight of acrylic ester, especially
n-butyl acrylate or 2-ethylhexyl acrylate, which if desired also
contain from 1 to 40% by weight of ethylene; vinyl ester-acrylic
ester copolymers with from 30 to 75% by weight of vinyl acetate,
from 1 to 30% by weight of vinyl laurate or vinyl esters of an
alpha-branched carboxylic acid, especially Versatic acid vinyl
ester, from 1 to 30% by weight of acrylic ester, especially n-butyl
acrylate or 2-ethylhexyl acrylate, which if desired also contain
from 1 to 40% by weight of ethylene; vinyl ester copolymers with
esters of maleic acid or fumaric acid such as diisopropyl,
di-n-butyl, di-t-butyl, di-ethylhexyl, methyl t-butyl esters, for
example, vinyl acetate copolymers with from 10 to 60% by weight of
one or more of said maleic/fumaric esters, which if desired also
contain ethylene or further copolymerizable vinyl esters such as
vinyl laurate or Versatic acid vinyl esters.
Examples of suitable protective colloids are polyvinyl alcohols
having a content of from 75 to 100 mol %, preferably from 78 to 95
mol %, of vinyl alcohol units and a Hoppler viscosity of from 10 to
25 mPas, polyvinylpyrrolidones having a molecular weight of
preferably from 5000 to 400,000; poly-saccharides in water-soluble
form such as starches (amylose and amylopectin), celluloses and
their carboxymethyl, methyl, hydroxyethyl, hydroxypropyl
derivatives, for example, hydroxyethylcelluloses having a degree of
substitution range of from 1.5 to 3; proteins such as casein, soya
protein, gelatin; ligninsulfonates, synthetic polymers such as poly
(meth) acrylic acid, poly (meth) acrylamide, poly-vinylsulfonic
acids and water-soluble copolymers thereof; melamine-formaldehyde
sulfonates, naphthalene-formaldehyde sulfonates, styrene-maleic
acid copolymers and vinyl ether-maleic acid copolymers.
Greatest preference is given to partially saponified polyvinyl
alcohols with from 78 to 95 mol % of vinyl alcohol units and a
Hoppler viscosity from 10 to 25 mPas (4% strength aqueous solution,
method of Hoppler in accordance with DIN 53015). Said partially
saponified polyvinyl alcohols can also be used, if desired, in a
mixture with further protective colloids from the abovementioned
group. The protective colloids are generally used in an amount of
from 1 to 15% by weight, based on the overall weight of the
monomers.
In addition to the protective colloid it is also possible if
desired to use emulsifiers in the polymerization. The amounts added
are in this case generally from 0.01 to 1% by weight, based on the
overall weight of the monomers. Suitable emulsifiers are, for
example, anionic surfactants, such as alkyl sulfates having a chain
length of 8 to 18 carbon atoms, alkyl and alkylaryl ether sulfates
having 8 to 18 carbon atoms in the hydrophobic radical and up to 40
ethylene or propylene oxide units, alkyl- or alkylarylsulfonates
having 8 to 18 carbon atoms, esters and monoesters of sulfosuccinic
acid with monovalent alcohols or alkylphenols. Suitable nonionic
surfactants are, for example, alkyl polyglycol ethers or alkylaryl
polyglycol ethers having from 8 to 40 ethylene oxide units.
The polymerization can be conducted batchwise or continuously, with
or without the use of seed latices, with all or individual
constituents of the reaction mixture being included in the initial
charge, or with the constituents, or certain constituents, of the
reaction mixture being included in part in the initial charge and
in part metered in subsequently, or by the metering process without
an initial charge. Normally, the makeup water is included in its
entirety in the initial charge together with the vinyl ester
monomer fraction. The dispersants, such as protective colloid and,
if appropriate, emulsifiers, can be included in whole or in part in
the initial charge.
The polymerization is preferably conducted within a temperature
range from 40.degree. C. to 80.degree. C. and initiated with the
methods commonly used for emulsion polymerization. In the case of
the copolymerization of gaseous comonomers such as ethylene the
reaction is operated under pressure; it is preferred to operate
under a pressure of up to 40 bar abs.
Initiation takes place by means of the water-soluble free-radical
initiators which are customary for emulsion polymerization, and
which are used preferably in amounts of from 0.01 to 1.0% by
weight, based on the overall weight of the monomers. Examples
thereof are ammonium persulfate and potassium persulfate, alkyl
hydroperoxides, such as t-butyl hydroperoxide; hydrogen peroxide.
If desired, said free-radical initiators can also be combined in a
known manner with from 0.01 to 0.5% by weight, based on the overall
weight of the monomers, of reducing agents. Suitable reducing
agents are, for example, formaldehyde sulfoxylate salts, sodium
bisulfite or ascorbic acid. The initiator can be included in whole
or in part in the initial charge or can be added continuously or in
stages in the course of the polymerization. In the case of redox
initiation, one or both redox catalyst components are preferably
metered during the polymerization.
The pH range desired for the polymerization, which generally lies
between 3 and 7, can be established in a known manner by means of
acids, bases or customary buffer salts, such as alkali metal
phosphates or alkali metal carbonates. For adjusting molecular
weight it is possible to add the commonly used regulators during
the polymerization, for example, mercaptans, aldehydes and
chlorinated hydrocarbons.
After the end of the polymerization, the dispersion can be
distilled, or stripped with steam or inert gas, in order to reduce
further the content of volatile constituents. These procedures are
known to the skilled worker and therefore require no further
explanation.
The polymerization is generally conducted so as to result in a
solids content of from 30 to 65% by weight, preferably from 40 to
50% by weight. The level of the solids content aimed at after the
end of the polymerization depends in this case on the viscosity of
the resulting dispersion, which should not exceed a value of from
10,000 to 15,000 mPas (Brookfield viscosity at 20 r).
The water-redispersible polymer powder which is added to the
polymer dispersion in order to increase the solids content has the
same composition as the polymer of the aqueous polymer dispersion
and is prepared with the same process. The drying of the dispersion
thus obtained can take place by means of spray drying, freeze
drying or fluidized bed drying. Preference is given to spray drying
in customary spray drying facilities, where atomization can take
place by means of single-, dual- or multi-substance nozzles or with
a rotating disk. The exit temperature is generally chosen within
the range from 55.degree. C. to 100.degree. C., preferably from
65.degree. C. to 90.degree. C., depending on the facility, the Tg
of the resin, and the desired degree of drying.
If desired, one or more of the abovementioned protective colloids
used in the polymerization can be added to the dispersion as an
atomization aid for spray drying. In the case of this process step,
preference is given to the addition of from 5 to 20% by weight of
protective colloid.
The water-redispersible polymer powder is incorporated into the
aqueous dispersion by means of conventional mixer-stirrer units.
The amount added depends on the target solids content. In general,
solids contents of from 50 to 75% by weight, preferably from 50 to
65% by weight, are established. The viscosity of the dispersions in
this case is from 20,000 to 60,000 mPas (Brookfield viscosity at 20
r).
The dispersions prepared in accordance with the invention are
suitable as adhesives for the adhesive bonding of porous substrates
such as wood, paperboard and paper. The adhesive dispersions are
particularly suitable as wood adhesives, wood flooring adhesives,
packaging adhesives and bookbinding adhesives.
For these applications, the adhesive dispersions can be modified
with the corresponding additives. Suitable additives are fillers
such as chalk or gypsum. It is also possible to add wetting agents,
dispersing agents, thickeners, defoamers and/or preservatives to
the adhesive dispersion.
Specifically for application as wood adhesive, in order to increase
the resistance to cold and hot water, it is possible to add the
crosslinking agents customary for this purpose, for example,
water-soluble metal salts of monobasic acids such as Al(III) and
Zr(IV) salts. It is preferred to add aluminum chloride and aluminum
nitrate.
With the procedure according to the invention it is possible to
obtain dispersions having a high solids content without the
occurrence of the disadvantages of the processes from the prior
art, such as long polymerization times, complex process regimes.
The performance properties of the dispersions are not adversely
affected by the procedure according to the invention. Above all,
the drastic reduction in the polymerization time brings
considerable economic advantages.
The examples which follow serve to further elucidate the
invention:
Comparative Example 1
Preparation of a Dispersion with a Solids Content of 50% by Weight
by the Water Dilution Process
In a 120 l reactor with automatic metering devices, temperature
regulation, anchor stirrer, jacket heat dissipation system and
reflux condenser, 25 kg of vinyl acetate were emulsified in 50 l of
an aqueous 10% polyvinyl alcohol solution. The polyvinyl alcohol
had a degree of hydrolysis of 88 mol % and a viscosity of 26 mPas
(4% strength aqueous solution). Subsequently, the mixture was
heated to 60.degree. C. and the polymerization was started using
the initiator system ascorbic acid/ammonium persulfate, with 0.1%
of acetaldehyde as a 10% strength aqueous solution being metered in
parallel with the initiator metering. About 10 minutes after the
onset of the polymerization, the vinyl acetate/water azeotrope came
to boiling, the temperature regulation and the initiator metering
being controlled in such a way that a moderate reflux of the
azeotrope over the reflux condenser was established. Thereafter, a
further 30 kg of vinyl acetate were metered in such that the
moderate reflux of the azeotrope was retained. After a reaction
time of about 6 hours, the polymerization was at an end.
The resulting dispersion had a solids content of about 57% and a
Brookfield viscosity of about 60,000 mPas at 20 r. In order to
establish the target solids content of 50%, the dispersion was
diluted with 15 l of water. A dispersion was obtained having a
solids content of 50% and a Brookfield viscosity of 15,000 mPas at
20 r.
EXAMPLE 1
Preparation of a Dispersion with a Solids Content of 50% by Weight
by the Powder Thickening Process
As in Comparative Example 1, in a 120 l reactor with automatic
metering devices, temperature regulation, anchor stirrer, jacket
heat dissipation system and reflux condenser, 25 kg of vinyl
acetate were emulsified in 50 l of an aqueous 10% polyvinyl alcohol
solution. The polyvinyl alcohol had a degree of hydrolysis of 88
mol % and a viscosity of 26 mPas (4% strength aqueous solution).
Subsequently, the mixture was heated to 60.degree. C. and the
polymerization was started using the initiator system ascorbic
acid/ammonium persulfate, with 0.1% of acetaldehyde as a 10%
strength aqueous solution being metered in parallel with the
initiator metering. About 10 minutes after the onset of the
polymerization, the vinyl acetate/water azeotrope came to boiling,
the temperature regulation and the initiator metering being
controlled in such a way that a moderate reflux of the azeotrope
over the reflux condenser was established. In contradistinction to
the procedure of Comparative Example 1, thereafter, a further 30 kg
of vinyl acetate were metered in together with 20 l of water such
that the moderate reflux of the azeotrope was retained. The
reaction time shortened to 3 hours.
The resulting dispersion had a solids content of about 48% and a
Brookfield viscosity of about 10,000 mPas at 20 r.
In order to establish the target solids content of 50%, the
dispersion was admixed with 5 kg of a dispersion powder which was
obtained by spray-drying an aqueous polymer dispersion which had
been prepared with the same process. To prepare the dispersion
powder, the polyvinyl acetate dispersion was diluted to a solids
content of 35%. Drying was carried out in a Nubilosa spray dryer at
an exit temperature of 820.degree. C. and a compressed-air pressure
upstream of the 2-substance nozzle of 4 bar.
Following addition of the dispersion powder, a dispersion was
obtained having a solids content of 50% and a Brookfield viscosity
of 15,000 mPas at 20 r.
Performance Testing
The dispersions obtained with the processes according to
Comparative Example 1 and Example 1 were tested for their
suitability as wood adhesives. For this purpose, the bond strength
was determined in accordance with DIN EN 204/205 and the heat
stability of the bonds in accordance with WATT 91.
Bond Strength in Accordance with DIN EN 204/205
The specimens were prepared in accordance with DIN EN 205. For this
purpose, in each case two beechwood panels each 5 mm thick, 130 mm
wide and 600 mm long were bonded to one another using the adhesive
dispersion to be tested, under pressure distributed uniformly over
the bond area, the bonded panels were divided into test specimens
each 150 mm long, and these specimens were stored in accordance
with DIN EN 204.
For the test for assignment to the stress group D2, the specimens
were stored, after gluing, for 7 days under standard climatic
conditions (23.degree. C., 50% atmospheric humidity), then for 3
hours in cold water at 23.degree. C. and finally for 7 days again
under standard climatic conditions.
Following storage, the test specimens were used to determine the
bond strength in a tensile shear test, the bonded test specimens
being pulled apart and loaded to fracture with a tensile testing
machine at a pulling speed of 50 mm/min. The maximum force
F.sub.max occurring at fracture was measured. The bond strength T
is calculated in accordance with DIN EN 205 from T=F.sub.max /A,
where A is the bonded test area in mm.sup.2.
The results of the testing of the bond strength are collated in
Table 1.
Testing of the Heat Stability in Accordance with WATT 91:
The specimens were produced and divided in accordance with EN 205
and stored for 7 days under standard climatic conditions.
Thereafter, they were stored for 1 hour at a temperature of
80.degree. C. in a heating cabinet preheated to 80.degree. C.
Immediately after the hot storage, the heat stability was
determined in the tensile shear test in analogy to DIN EN 205, the
bonded test specimens being pulled apart and loaded to fracture
with a tensile testing machine at a pulling speed of 50 mm/min. The
maximum force F.sub.max occurring at fracture was measured. The
heat stability is calculated from Fmax/A, where A is the bonded
test area in mm.sup.2.
The results of testing the heat stability are likewise collated in
Table 1.
TABLE 1 Comparative Example Example 1 Example 1 Bond strength
(N/mm.sup.2): 9.5 9.5 D2 value Heat stability (N/mm.sup.2): 7.0
7.0
* * * * *